円周率。

mc.js

(function () {
  var randint = function (min, max) {
    if (max == null) {
      max = min;
      min = 0;
    }

    return Math.floor(Math.random() * (max - min + 1) + min);
  };

  var plot = function (ctx, x, y, color) {
    if (color == null) {
      color = 'rgb(0, 0, 0)';
    }

    ctx.fillStyle = color;
    ctx.fillRect(x - 2, y, 5, 1);
    ctx.fillRect(x, y - 2, 1, 5);
  };

  var judge = function (x0, y0, r, x1, y1) {
    return (x0 - x1) * (x0 - x1) + (y0 - y1) * (y0 - y1) < r * r;
  };

  (function init(num) {
    var cv, ctx, div, txt;
    var red, blue;
    var time;
    var i, k;

    cv = document.createElement('canvas');

    div = document.createElement('div');
    txt = document.createTextNode('');

    div.style.fontSize = 'small';
    div.style.fontFamily = 'Consolas, monospace';

    cv.width = cv.height = 200;
    cv.style.border = '1px solid rgb(0, 0, 0)';

    ctx = cv.getContext('2d');

    red = 'rgb(127, 0, 0)';
    blue = 'rgb(0, 0, 127)';

    i = 1;
    k = 0;

    time = setInterval(function () {
      var x, y, j;

      x = randint(200);
      y = randint(200);

      j = judge(0, 200, 200, x, y);

      if (j) {
        k++;
      }

      plot(ctx, x, y, j ? blue : red);

      txt.textContent = 'π: ' + 4 * k / i;

      if (i >= num) {
        clearInterval(time);
      }

      i++;
    }, 1000 / 60);

    div.appendChild(txt);
    document.body.appendChild(div);

    document.body.appendChild(cv);
  })(10000);
}).call(this);